This invention relates to flywheels adapted for the storage of energy and more particularly to cross-ply composite flywheels having reinforced rim portions.
To overcome various disadvantages such as high weight and potentially dangerous fracture mechanics associated with metallic flywheels, composite flywheels have been developed. These composite flywheels are normally fabricated from a multiplicity of glass or similar fibers disposed in a matrix or binder of epoxy or other suitable resin. Such composite flywheels are generally of a high strength-to-density ratio and therefore, to be able to store sufficient amounts of energy, may be required to rotate at extremely high speeds such as tens of thousands of revolutions per minute. These prior art composite flywheels have for the most part been formed by a circumferential distribution of all the fibers within the binder or matrix. Since, when a body rotates it is subject to stresses due to centrifugal force in a radially outward direction, such prior art composite flywheels must carry this centrifugal stress in directions normal to the axes of the fibers from which it is formed, placing the resin matrix in tension. Analysis has shown that such radial stresses are greatest in magnitude at locations approximately mid-way between the center and outer edge of the flywheel. Usually, epoxy and other resins employed with composite flywheels are relatively weak when loaded in tension as compared to the strengths of the fibers. Therefore, a high rotational speed of such a composite flywheel can cause the flywheel to shred, breaking apart in the matrix along circumferential lines between the fibers.
To overcome such deficiencies of these prior art composite flywheels having a complete circumferential distribution of fibers, cross-ply composite flywheels have been developed. Such cross-ply composite flywheels comprise a plurality of overlying fiber layers embedded in a monolithic matrix of binder material. The fibers of each layer are parallel to each other and extend in radial and chordal directions. The fibers of each successive layer are oriented at a single predetermined angle to those of a preceding adjacent layer. Preferably, the arrangement is such that at least four layers of fibers are disposed between any two layers of fibers in which the fibers of both such layers extend substantially parallel to each other. By this structure the cross-ply composite flywheel is made to exhibit a high strength-to-weight ratio. The preferred cross-ply composite flywheel construction is described and claimed in U.S. patent application Ser. No. 706,896, filed on July 19, 1976 in the name of Burton D. Hatch, entitled entitled "Cross-Ply Composite Flywheel" and now U.S. Pat. No. 4,102,221.
Prior to the invention described in the U.S. Pat. No. 4,102,221, composite materials were known which were formed from unified laminar constructions, each lamination comprising a multiplicity of parallel fibers embedded in a matrix of binder material and overlying an adjacent lamination such that the fibers of adjacent laminations are oriented at angles of either 60.degree. or 90.degree. with respect to each other. However, flywheels formed from such prior art materials of these laminar constructions and particular angular orientations exhibit significant variations in load carrying abilities throughout their mass. For example, in a flywheel having a 60.degree. orientation between fibers of adjacent layers, if a first point is capable of carrying a particular value of centrifugal loading at a given radial strain (i.e. displacement), a second point at the same radius but displaced by 30.degree. from the first point will be able to accommodate only 87% of that loading at the same radial strain. Such a variation in strength and stiffness throughout a flywheel has the effect of introducing shear stresses and severely limiting the load carrying ability of the flywheel and therefore limits the energy storage capabilities of the flywheel.
As described in U.S. Pat. No. 4,102,221 (principally in connection with its FIGS. 5 and 6), in flywheels thereof employing an angular offset of approximately 70.degree. to approximately 110.degree. the fibers of one layer are in near perpendicular orientation with respect to fibers of an adjacent layer. In this construction a major portion of a centrifugal force acting in a direction approximately perpendicular to the axes of a fiber in a given layer is transmitted to an adjacent fiber of an adjacent layer; the adjacent layer accommodates the transmitted force by a resultant axial force, i.e. in the direction of the fiber's greatest strength. With small angles such perpendicular forces on a given fiber could not be so transmitted to adjacent fibers of adjacent layers.
It has been found that the energy storage capability of the cross-ply composite flywheel may be improved upon in certain respects. Each composite layer of such a flywheel, at locations on the outer edge thereof, includes fibers oriented such that the direction of the fibers is perpendicular to a radius. Therefore, these composites are centrifugally loaded in a direction approximately 90.degree. from the axes of the fibers, a direction in which the composite elements are relatively weak as compared to their strengths when axially loaded. The cross-ply configuration allows loads in such fibers disposed in a central layer to be effectively transferred to radially directed fibers in layers adjacent each side of the central layer. However, such fibers when disposed in an axially outer layer (comprising a major surface of the flywheel) are only able to transfer this centrifugal loading in a single axial direction. In this instance the numbers of radially directed fibers to which such loading may be transferred is severly limited. Therefore, the possibility of rupture of the radially outer (chordal) fibers disposed in an axially outermost layer or the failure of the matrix or binder between such fibers due to centrifugal loading may limit the maximum speed of the flywheel and hence the maximum amount of energy which may be stored by the flywheel. By the present invention, this problem is overcome and, thereby the maximum allowable speed of rotation and thus the energy storing capability of the cross-ply composite flywheel are increased.
Therefore, it is an object of the present invention to provide a reinforced cross-ply composite flywheel capable of storing increased amounts of energy within limited constraints of weight and volume.
It is another object of the present invention to provide a reinforced composite cross-ply flywheel wherein the risk of delamination or fracture under high rotational speeds is minimized.